This invention relates to a novel process for preparing granular, cross-linked starches. More particularly, this invention relates to a process for preparing granular, acetal cross-linked starches having substantially removable cross-linkages.
It is well known in the art to react starch bases under a variety of reaction conditions with cross-linking agents to obtain cross-linked starches. The granules of a cross-linked starch have been toughened so that they are more resistant to rupturing during cooking than ordinary starch granules. Cross-linked starches may exhibit a markedly reduced tendency to swell or gelatinize and generally display a comparatively short, non-cohesive consistency after cooking. The degree of cross-linking can be controlled and varied over a wide range so as to produce starches in which the tendency of the swollen granules to rupture is decreased through successive stages to starch products in which the swelling of the granules is so highly restrained that they will not swell noticeably when cooked in boiling water. A general discussion of cross-linking of starches and the reagents used therefore may be found in such sources as "Starch: Chemistry and Technology", by R. L. Whistler and E. F. Paschall (ed.), Vol. II (New York and London: Academic Press, 1967). Typical cross-linking agents employed in the preparation of starches for food and industrial use include, for, example, epichlorohydrin, phosphorus oxychloride, water-soluble metaphosphates, cyanuric chloride, aldehydes, and the like.
It has been found that prior cross-linking of a granular starch base allows the starch to be further chemically modified while remaining granular in form under conditions which, in the absence of cross-linking, would produce a non-granular starch derivative. Depending on the ultimate use of the cross-linked starch, it may be desirable to remove the cross-linkages from the final starch product after the intended function has been accomplished. For example, cross-linking may be used to maintain the granules of a starch undergoing a substitution reaction. The product can be prepared in water and washed with water, after which the cross-linkages may be removed with a gelatinized starch product resulting therefrom.
Propiolate esters have not been used in the prior art as cross-linking agents per se. A process for preparing certain starch derivatives (e.g., carboxylatevinyl starch ethers) using propiolate esters is described in U.S. Pat. No. 3,022,288, but concurrent cross-linking of the starches is not reported to occur. Furthermore, the reaction described in the patent is conducted in non-aqueous media with highly alkaline conditions being required, and most of the resultant starch products are non-granular.
Accordingly, it is an object of the present invention to provide a novel process for preparing granular, cross-linked starches.
It is another object to provide a process for preparing granular, acetal cross-linked starches in aqueous neutral or alkaline medium.
It is a further object to provide a novel process for preparing granular, acetal cross-linked starches having cross-linkages which are substantially removable under acidic conditions.